1. Field of the Invention
This invention relates to improvements in the cementitious properties of granulated, ferrous blast furnace slags used to produce cementitious materials. Specifically, it relates to a process for mixing a source of calcium oxide (CaO) with molten ferrous blast furnace slag, such that the hydraulic reactivity of the slag after granulating is enhanced. When such slag is ground, cementitious properties that are the same as or similar to those of more finely ground cementitious materials can be achieved at a lower specific surface area, thereby saving production and energy costs. Moreover, improved properties can be achieved by grinding such slag to the same or similar specific surface area as that of a ground, granulated blast furnace slag produced without the addition of a source of CaO. The invention further relates to a system for performing this addition.
2. Description of the Related Art
Portland Cement is the most widely used cementitious material. It is manufactured by heating a finely ground and controlled blend of limestone (or another source of CaO, such as chalk) with an argillaceous material such as shale or clay to a temperature of about 3180.degree. F. (1500.degree. C.). When necessary, other sources of silica (SiO.sub.2), such as sand; alumina (Al.sub.2 O.sub.3), such as bauxite; and ferric oxide (Fe.sub.2 O.sub.3), such as pyrites, may also be added. After it is cooled, the resultant clinker is mixed with gypsum (or a mixture of anhydrite and gypsum) and ground to a fine powder.
There are a number of different processes for making cementitious materials. In a wet process, the mixture of materials is ground with water to produce a slurry which is then fed to a rotary kiln. In this process, the fuel consumption in the kiln is generally high due to the need to dry, as well as to heat the materials. In a traditional dry process, the materials are ground dry to form a powder which is then fed to a kiln. This process generally has a lower fuel consumption than wet processes. Variants of these processes include a semi-wet process, in which water is partially removed from the slurry by filtration to form a cake of ground materials which then is fed to the kiln, or the Lepol process, in which a dry powder is first nodulized with the addition of some water after which the nodules are fed to the kiln.
In a suspension preheater process, the materials are ground dry to form a powder which is passed through a series of cyclones before it enters the kiln. In these cyclones, the powder mixes with waste gases from the kiln and is gradually heated, such that the powder may be reach a temperature of about 1590.degree. F. (750.degree. C.). As a consequence of this preheating, the fuel consumption in this process may be lower than in the processes described above.
The suspension preheater process may be modified by the addition of a combustion chamber between the cyclones and the rotary kiln in which about 50 percent of the total fuel required to produce a clinker can be fired. This process is known as a precalciner process. In this process, the powder may be heated to about 2014.degree. F. (950.degree. C.) before it enters the kiln. An advantage of this process is that a higher output may be attained for a given kiln size. Even in the more thermally efficient suspension preheater and precalciner processes, however, the clinker forming step is usually the most energy intensive in processes for making cementitious materials.
When the finely ground blend of materials is heated in any of the above processes, the argillaceous materials usually begin to de-hydrate at about 1272.degree. F. (600.degree. C.), and the calcareous materials may decarbonate at about 1908.degree. F. (900.degree. C.). At about 2756.degree. F. (1300.degree. C.) the mixture usually begins to melt, so that by about 3180.degree. F. (1500.degree. C.) about 25 percent of the mixture is molten. This liquid encourages the formation of clinker and promotes chemical reactions between the CaO, SiO.sub.2, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3, to form four chemical compounds commonly referred to as clinker minerals, which are described in Table I. All percentages expressed herein are by weight.
TABLE I ______________________________________ Short- Typical Levels in hand Ordinary Portland Chemical Nomen- Mineral Cement (OPC) Formulae clature Name (%) ______________________________________ 3CaO.SiO.sub.2 C.sub.3 S Alite 45-70 2CaO.SiO.sub.2 C.sub.2 S Belite 6-30 3CaO.Al.sub.2 O.sub.3 C.sub.3 A Aluminate 5-12 4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3 C.sub.4 AF Ferrite 5-12 ______________________________________
When clinker containing these minerals is ground to a fine powder and mixed with water, the aluminate reacts rapidly causing the mixture to set. For this reason, clinker may be ground with about five (5) percent gypsum (or another appropriate source of calcium sulfate (CaSO.sub.4)) to control this reaction, so that grouts, mortars, or concretes prepared from cementitious materials can be placed and compacted (or poured) before hardening commences.
After setting, the cementitious material gains strength as the calcium silicate compounds hydrate. Alite is the main strength giving mineral in Portland Cement contributing to both early and late strengths, whereas belite is less reactive and only contributes to strengths typically fourteen (14) days after placement (or pouring). Both minerals react with water to form a non-crystalline, calcium silicate hydrate or C--S--H gel. In this gel, the ratio of CaO to SiO.sub.2 (hereinafter the "C/S Ratio") is less than that in the unhydrated clinker minerals, and the CaO liberated as the minerals hydrate combines with water to form calcium hydroxide (CA(OH).sub.2).
Cementitious material users are particularly interested in setting and strength development characteristics, and for this reason, minimum strengths and both minimum and maximum setting times are specified in the ASTM (American Society For Testing and Materials) C150 standard specification for Portland Cements. In addition to the clinker minerals described in Table I, there are a number of minor components which form in the clinker from impurities in the raw materials or fuel. These minor components can influence both the clinker forming process and the hydraulic reactivity and cementitious properties of the resultant cementitious material. The level of alkalis, notably K.sub.2 O and Na.sub.2 O, present in Portland Cement may be of particular concern. If cementitious materials are combined with aggregates containing reactive SiO.sub.2 to make concrete, the alkalis from the cementitious material may react with this SiO.sub.2 to form an expansive alkali silica gel which can lead to the cracking and break up of the concrete structure. Detecting and avoiding the use of aggregates containing reactive SiO.sub.2 is difficult. Therefore, in order to avoid this problem, cementitious materials with a specified low alkali content are commonly used. Thus, a maximum equivalent Na.sub.2 O of about 0.60 percent is included as an optional limit in the ASTM C150.
One way of safely using cementitious materials containing more than about 0.60 percent equivalent Na.sub.2 O with aggregates containing reactive SiO.sub.2, and of avoiding excessive expansion while at the same time reducing the total energy consumption to manufacture the cementitious materials, is to mix the Portland Cement with other materials which are not cementitious by themselves, but which are capable of reacting with the alkalis and Ca(OH).sub.2 liberated, as the Portland Cement hydrates to form additional cementitious hydrates. Suitable materials for this purpose include pozzolanic materials, such as power station fly ash and certain types of naturally occurring volcanic material, or latently hydraulic materials, such as ground, granulated blast furnace slag.
Slag, as used herein, refers to the material that remains after the smelting of a metallic ore, a process which entails the reduction of the ore to a molten state. Smelting of iron ore generally involves the combination of iron ore; a source of carbon, generally coke; and a flux, such as limestone, in a blast furnace. The terms "ferrous slag" or "ferrous blast furnace slag" refer to the slag that remains after the smelting of iron ore.
Pozzolanic materials are generally low in CaO and contain SiO.sub.2 and Al.sub.2 O.sub.3 in an active--usually vitreous--form, which react with the Ca(OH).sub.2 released by the hydrating Portland Cement. Latently hydraulic materials contain sufficient CaO, SiO.sub.2, and Al.sub.2 O.sub.3 to form their own calcium silicate and aluminate hydrates. The reactions may only be activated in suitably alkaline conditions, such as are achieved in the presence of hydrating Portland Cement or by adding small amounts of alkalis, such as sodium hydroxide or sodium carbonate, to the latently hydraulic material.
Consequently, pozzolana and latently hydraulic materials, even when mixed with Portland Cement, do not react as fast as Portland Cement and as a result, contribute more to the late strength than to the early strength of a cementitious material. An advantage of their lower early reactivity, however, is that pozzolana and latently hydraulic materials produce less heat than hydrating Portland Cement during the early reaction stages. If only Portland Cement were to be used, the heat produced in large structures could subject the structure to excessive thermal stresses during the initial hardening and result in crack formation.
In addition to the advantages outlined above, mixing Portland Cement with ground, granulated blast furnace slag can improve the workability of concrete, mortar, or grout, thereby making it easier to place and compact (or pour). Mixing can also reduce the permeability of hardened concrete, mortar, or grout, thereby improving its durability. At high percentage slag mixtures, a resultant concrete, mortar, or grout may have sulfate resisting properties equal to or better than an ASTM Type V cement, as defined in ASTM C150.
Granulated ferrous blast furnace slag may be mixed with Portland Cement in various ways. The slag may be interground with the Portland Cement clinker and gypsum to produce a cementitious material. This is a common practice in continental Europe. Alternatively, it may be ground alone and then mixed with Portland Cement as is common practice in the United States of America and the United Kingdom. An advantage of having the ground, granulated blast furnace slag available separately is that it can be activated by the addition of alkalis, such as sodium hydroxide or sodium carbonate, without any Portland cement, for certain specific applications such as soil stabilization, as described in U.S. Pat. No. 4,761,183, and the solidification of mud for well cementing operations, as described by Cowan et al. in Paper SPE 24575 presented at the 67th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers held in Washington, D.C. in October 1992.
In order to acquire hydraulic properties, blast furnace slag must be quenched rapidly to preserve the molten slag in a vitreous state. Two processes that are commonly used are granulation and pelletization. In granulation, the slag is quenched by the injection of a large quantity of water under pressure into the slag, i.e., water granulation. If the slag temperature prior to quenching is above the liquidus, water granulation yields a wet sand like material with a high degree of vitrification. The water is injected under pressure and at a sufficiently low temperature to quench the slag and break up the molten slag into small pieces. Because of its increased surface area, the molten slag cools quickly. In a pelletization process, the molten slag is directed onto a spinning drum together with a small quantity of water. The slag is then projected into the air and solidifies as small, round, and frequently hollow pellets. As a consequence of the slower quench rate, the degree of vitrification of pelletized slags is normally lower than that of granulated slags. If permitted to cool slowly or allowed to form large, slow cooling pieces, molten slag may crystalize and exhibit little or no cementitious qualities.
Despite the advantages gained by mixing Portland Cement with ground, granulated blast furnace slag, strength development remains an important property, and for this reason, it has been traditional in the United States of America to compare the strength measured in psi after about seven (7) and twenty-eight (28) days curing of a 50/50 blend of Portland Cement and ground, granulated blast furnace slag with unblended Portland Cement, as outlined in ASTM C989. This comparison has been expressed as a slag activity index (SAI). This ratio of strengths permits the comparison of the hydration rates of various types of cementitious materials.
The glass content and the composition of the slag, however, will also provide a guide to its hydraulic reactivity, and it is also common to consider the C/S Ratio, or the ratio of the combined CaO, MgO, and Al.sub.2 O.sub.3 contents to the SiO.sub.2 content (hereinafter the "Hydraulic Index"). In general, slag with the highest glass content, the highest C/S Ratio, and Hydraulic Index is preferred for use with Portland Cement. Nevertheless, the higher the C/S Ratio and Hydraulic Index the higher the slag melting temperature and the more difficult it is to quench the molten slag to a vitreous mass, without formation of crystalline phrases. Because slag is a by-product from the iron smelting (and steel-making) process, its composition is largely dictated by the composition of the iron ore, coke, and other raw materials and the efficiency of furnace operation.
Ferrous slag produced in the United States of America, however, usually has a C/S Ratio of about 1. Nevertheless, as the CaO content and thus, the C/S Ratio of the slag increase, the viscosity of the slag also increases. As the viscosity of the slag increases, however, the efficiency of operation of the blast furnace may decline. Further, sources of CaO may be relatively expensive components in the smelting of iron. Iron smelters, therefore, may seek to limit the amount of any source of CaO that they add to the smelting process, so that they can minimize the costs of smelting and maximize the efficiency of their furnaces.
For example in the United States of America, iron smelters will commonly monitor the Base Number of the molten slag produced by their furnaces. EQU Base Number=(CaO+MgO)/SiO.sub.2.
When the Base Number exceeds about 1.55 and the C/S Ratio exceeds about 1.25, the viscosity of the slag may have significant adverse effects on blast furnace operations and efficiency. In the United States of America, where competition in the iron and steel industry is intense, iron smelters generally operate their furnaces to produce slag with relatively low C/S Ratios and Base Numbers. In other countries, however, C/S Ratios or Base Numbers may be higher due to furnace and raw material differences. As a consequence, the hydraulic reactivity of blast furnace slag may be below its potential and is often variable. To produce a suitable ground, granulated blast furnace slag from such slag, it is necessary to finely grind the slag to achieve very high surface areas to ensure it conforms with ASTM C989 and to vary the fineness of the grind, so that the variability of the product is reduced to a level acceptable to users. This results in high energy costs to grind the slag, lower output, and higher testing and manpower costs to monitor the slag quality and to adjust slag grinding operations.
Applicant has developed a process and system for incorporating a source of CaO into molten slag, such that the C/S Ratio of the slag rises, but the slag can still be successfully granulated to produce a highly vitreous mass. Such slag exhibits improved hydraulic reactivity. Therefore, Applicant's process permits less finely ground, granulated slags to be used to produce cementitious materials possessing qualities previously available only from more finely ground, granulated slags; thus reducing energy costs. Alternatively, Applicant's process permits the production of ground granulated slags with improved properties over previous granulated slags which were ground to the same fineness. With such slags, it is possible in certain applications to mix more ground, granulated blast furnace slag with the Portland Cement and still produce high quality cementitious materials.